1. Field of the Invention
The present invention relates to a projection display apparatus for projecting by rays of light an optical image of a picture produced in a light valve onto a screen with a projection lens to obtain an enlargement of the picture.
2. Description of the Prior Art
It is well known to modulate light with a relatively small-sized light valve which has produced therein an optical image of a picture according to a video signal and project an enlarged image of the picture with a projection lens onto a screen for display of an enlarged picture. According to such a projection display apparatus, the resolution of the projected picture largely depends on the resolution of the light valve and a high-intensity light source enables to provide a greater output of light. Thus, the projection display apparatus employing a light valve of high resolution can provide a picture of high brightness and high resolution although a display area of the light valve is rather small. Also, a liquid crystal panel is successfully used as a light valve nowadays. For instance, a method of providing a color projected picture with the use of three liquid crystal panels is described in the report "LCD full-color video projector" by Morizumi et al. on Page 375 of SID 86 Digest (1986). The fundamental arrangement of the optical system of Morizumi et al. is shown in FIG. 11.
As shown in FIG. 11, a lamp 1 radiates a light ray having three primary colors of red, green and blue. The radiant light ray from lamp 1 is converted to a parallel ray by a condenser lens 2 and a concave mirror 3, passes through a heat absorbing filter 4, and enters a component color separator 5. The component color separator 5 includes a flat-plate type red-reflecting dichroic mirror 6 and a couple of flat-plate type half-separated blue-reflecting dichroic mirrors 7 and 8 which are arranged at right angles so as to form an X shape. The dichroic mirror surfaces 9 and 10 of their respective mirrors 7 and 8 are on a same plane. Light transmitted to the component color separator 5 is then separated into three colors of red, green and blue. Red color light is reflected by plane mirrors 11 and 12 to enter a red color liquid crystal panel 15. Green color light is straightly transmitted to a green color liquid crystal panel 16. Blue color light is reflected by plane mirrors 13 and 14 to enter a blue color liquid crystal panel 17. The liquid crystal panels 15, 16 and 17 produce optical images in red, green and blue colors respectively as variations in the transmittance of light according to a video signal. The light outputs from the liquid crystal panels 15, 16 and 17 are transformed by a light combiner 18 into a composite light flux to provide a color picture substantially developed at the position of the green color liquid crystal panel 16. An enlarged image of the color picture is then projected with a projection lens 19 onto a screen 20. The light combiner 18 is a prism type dichroic mirror comprising four rectangular prisms 21, 22, 23 and 24 which are all joined together at joint surfaces 25 and 26 both coated with red reflecting dichroic multi-layer films and at joint surfaces 27 and 28 both coated with blue reflecting dichroic multi-layer films so that their respective red and blue reflecting dichroic surfaces can intersect each other at right angles to form an X shape in cross section.
The optical arrangement shown in FIG. 11 has a feature that each of the component color separator 5 and the light combiner 18 comprises dichroic mirrors provided in X-shaped arrangement. Such an arrangement in which a pair of dichroic mirrors are provided in X-shaped arrangement for a component color separator system or a light combiner system, is disclosed in U.S. Pat. No. 2,642,487, "Component color separator", to Schroder.
Another projection display apparatus incorporating an optical arrangement as shown in FIG. 12 is described in the article of "High-resolution full-color video projector with poly-Si TFT array light valves" by Aruga et al., on Page 75, SID 87 Digest (1987). This apparatus has a component color separator different in arrangement from that of FIG. 11.
As shown in FIG. 12, a component color separator 30 includes a flat-plate type blue reflecting dichroic mirror 31 and a flat-plate type green reflecting dichroic mirror 32 which are arranged in parallel to each other. The dichroic mirror surfaces 33 and 34 of their respective mirrors 31 and 32 are positioned such that their respective normal lines 35 and 36 are arranged at angles of 45 degrees to the optical axis 37 of a condenser lens 2. As light enters the component color separator 30, blue light is reflected by the blue reflecting dichroic mirror 31 and then by a plane mirror 38 to enter a blue liquid crystal panel 17. Simultaneously, green color light is reflected by the green reflecting dichroic mirror 32 and then directed to a green liquid crystal panel 16. Red color light transmitted by dichroic mirrors 31 and 32 is reflected by plane mirrors 39 and 40 to enter a red liquid crystal panel 15.
Each of the projection display apparatuses shown in FIGS. 11 and 12 includes a single projection lens so as to easily change the size of a projected picture or the distance between the projection lens 19 and the screen 20. Also, the light combiner 18 has the X-shaped arrangement of the dichroic mirror surfaces, thus permitting space saving in the optical system.
In either of the arrangements shown in FIGS. 11 and 12, the lengths of the optical paths of the three distinct colors from the condenser lens 2 to the respective liquid crystal panels 15, 16 and 17 are not the same. Particularly as shown in FIG. 11, the optical path of the green light is shorter than the optical paths of the red and blue lights. In FIG. 12 the optical paths of the blue and green lights are each shorter than the optical path of the red light. Generally, a light transmitted from the condenser lens 2 will diffuse more as its optical path extends further. This results in a low efficiency in the use of light of a particular color having a long optical path. For white balance in the projected picture, a ratio of the illumination between red, green and blue colors should e kept optimum, e.g. by inserting an attenuation filter into an optical path of a light of a certain color. The optical efficiency of the entire system depends on : color light lacking in illuminance to satisfy the optimum illuminance ratio between red, green and blue colors. Accordingly, both the arrangements shown in FIGS. 11 and 12 have a disadvantage that the efficiency of light n action is low and thus, the output light power will be small.
For the purpose of solving such a problem, there has been proposed by Sonehara a method employing a light guide which has a mirror surface on the inner surface thereof and is arranged along a longer optical path for restricting the diffusion of light by reflection by the mirror surface, as shown in Japanese Unexamined Patent Publication No. 62-237485 (1987). However, improvement in the optical efficiency is made at the peripheral area of a projected picture but not at the central area.
Another method of improving the optical efficiency is developed by Ono in which the diffusion of light is prevented along a longer optical path by converging light rays with a single condenser lens disposed just in the rear of a component color separator. This arrangement still has a disadvantage that a projected picture is less bright in the peripheral area than in the central area.
Furthermore, another projection display apparatus has been proposed by Ledebuhr in which three reflection type liquid crystal light valves are employed for developing a color picture, as described in the report of "Full-color single-projection-lens liquid crystal light-valve projector", on Page 379 of SID 86 (1986). The apparatus includes a 1:1 magnification telecentric relay lens system arranged along a longer optical path so that a surplus length of the optical path can be eliminated equivalently. This relay lens system will be effective with rays of incident light parallel to the optical axis. However, light rays other than the parallel light rays are shaded by an aperture disposed across the optical path or, even if transmitted through the aperture, the rays can hardly be projected from an output side relay lens effectively. Accordingly, the relay lens system is not much suitable for improvement of the optical efficiency although applicable to the diffusion of light.
As set forth above, the conventional projection display apparatus allows a light having a long optical path among red, green and blue lights to diffuse as it runs along the optical path, thus providing a less efficiency in the use of light. In each prior art method of improving the optical efficiency for prevention of a reduction in light energy, the optical efficiency cannot substantially be improved throughout the area of a projected picture and thus, a problem arises that there is a difference in brightness between the central and peripheral areas of the picture. It is understood that the greater the power of the light source is made, the more the light energy can be increased. This however causes the apparatus to become large in size and power consumption.